U.S. patent number 11,092,223 [Application Number 15/322,640] was granted by the patent office on 2021-08-17 for dual-type strain wave gearing.
This patent grant is currently assigned to HARMONIC DRIVE SYSTEMS INC.. The grantee listed for this patent is HARMONIC DRIVE SYSTEMS INC.. Invention is credited to Jun Handa, Yoshihide Kiyosawa, Noboru Takizawa, Xin Yue Zhang.
United States Patent |
11,092,223 |
Handa , et al. |
August 17, 2021 |
Dual-type strain wave gearing
Abstract
An externally toothed gear of a dual-type strain wave gearing is
provided with first and second external teeth having different
teeth numbers, and a gap formed between these teeth as a cutter
clearance area for tooth cutters. The maximum width L1 of the gap
is 0.1 to 0.3 times the width L of the externally toothed gear. The
depth from the tooth top land of the first external teeth to the
deepest part of the gap is 0.9 to 1.3 times the depth of the first
external teeth, and the depth from the tooth top land of the second
external teeth to the deepest part of the gap is 0.9 to 1.3 times
the depth of the second external teeth. The tooth bottom fatigue
strength of the externally toothed gear provided with differing
numbers of first and second external teeth is increased.
Inventors: |
Handa; Jun (Azumino,
JP), Kiyosawa; Yoshihide (Azumino, JP),
Takizawa; Noboru (Azumino, JP), Zhang; Xin Yue
(Azumino, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
HARMONIC DRIVE SYSTEMS INC. |
Tokyo |
N/A |
JP |
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Assignee: |
HARMONIC DRIVE SYSTEMS INC.
(Tokyo, JP)
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Family
ID: |
55162910 |
Appl.
No.: |
15/322,640 |
Filed: |
July 3, 2015 |
PCT
Filed: |
July 03, 2015 |
PCT No.: |
PCT/JP2015/069242 |
371(c)(1),(2),(4) Date: |
April 19, 2018 |
PCT
Pub. No.: |
WO2016/013378 |
PCT
Pub. Date: |
January 28, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190203819 A1 |
Jul 4, 2019 |
|
Foreign Application Priority Data
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|
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|
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Jul 23, 2014 [JP] |
|
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JP2014-149370 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16H
55/0833 (20130101); F16H 49/001 (20130101); F16H
1/32 (20130101); F16H 55/08 (20130101) |
Current International
Class: |
F16H
1/32 (20060101); F16H 55/08 (20060101); F16H
49/00 (20060101) |
Field of
Search: |
;74/640 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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01-108441 |
|
Jan 1989 |
|
JP |
|
H01-91151 |
|
Jun 1989 |
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JP |
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H02-275147 |
|
Nov 1990 |
|
JP |
|
2011-112214 |
|
Jun 2011 |
|
JP |
|
2009/157607 |
|
Dec 2009 |
|
WO |
|
WO 2013/084538 |
|
Jun 2013 |
|
WO |
|
WO 2014/155791 |
|
Oct 2014 |
|
WO |
|
Other References
International Search Report (Form PCT/ISA/210) dated Sep. 29, 2015,
by the Japanese Patent Office in corresponding International
Application No. PCT/JP2015/069242 and partial English Translation
(1 page). cited by applicant .
International Search Report (Form PCT/ISA/237) dated Sep. 29, 2015,
by the Japanese Patent Office in corresponding International
Application No. PCT/JP2015/069242. cited by applicant.
|
Primary Examiner: Joyce; William C
Attorney, Agent or Firm: Buchanan Ingersoll & Rooney
PC
Claims
The invention claimed is:
1. A strain wave gearing comprising: a rigid first internally
toothed gear formed with first internal teeth; a rigid second
internally toothed gear formed with second internal teeth, the
second internally toothed gear being disposed so as to be coaxially
aligned in parallel with the first internally toothed gear; a
flexible externally toothed gear in which first external teeth
capable of meshing with the first internal teeth and second
external teeth capable of meshing with the second internal teeth
are formed in an outer-peripheral surface of a radially flexible
cylindrical body, the second teeth differing in number from the
first teeth, and the externally toothed gear being disposed
coaxially inside the first and second internally toothed gears; and
a wave generator for flexing the externally toothed gear in an
ellipsoidal shape to cause the first external teeth to partially
mesh with the first internal teeth and to cause the second external
teeth to partially mesh with the second internal teeth, wherein a
gap is formed between a tooth-trace-direction inner-end surface of
the first external teeth and a tooth-trace-direction inner-end
surface of the second external teeth, the gap having a prescribed
width along a tooth trace direction, and the gap having a deepest
part along a tooth depth direction at a tooth-trace-direction
central portion; and wherein the wave generator has a first wave
bearing comprising a ball bearing for supporting the first external
teeth, and a second wave bearing comprising a ball bearing for
supporting the second external teeth; 0.1L<L1<0.3L, where L
is a width from a tooth-trace-direction outer end of the first
external teeth to a tooth-trace-direction outer end of the second
external teeth, and L1 is a maximum width of the gap along a tooth
trace direction; and 0.9h1<t1<1.3h1 0.9h2<t2<1.3h2 and
t(1)<t(2), where h1 is a tooth depth of the first external
teeth, h2 is a tooth depth of the second external teeth, t1 is a
distance from a tooth top land of the first external teeth to the
deepest part of the gap, t2 is a distance from a tooth top land of
the second external teeth to the deepest part of the gap, t(1) is a
rim wall thickness of the first external tooth, and t(2) is a rim
wall thickness of the second external tooth, and bearing-ball
centers of the first wave bearing and the second wave bearing are
located at positions that are equidistant, along the tooth trace
direction, from a tooth-trace-direction center of the gap; where an
inter-ball-center distance Lo is a distance between the
bearing-ball centers of the first and second wave bearings, and the
inter-ball-center distance is set so as to increase correspondingly
with an increase in the maximum width L1 of the gap, and to satisfy
a relationship 0.35L<Lo<0.7L.
2. The strain wave gearing according to claim 1, wherein a number
of the first external teeth differs from a number of the first
internal teeth, and a number of second external teeth differs from
a number of second internal teeth.
3. The strain wave gearing according to claim 1, wherein a number
of first external teeth is less than a number of first internal
teeth, and a number of first internal teeth and a number of second
internal teeth are equal to each other.
4. The strain wave gearing according to claim 1 wherein the wave
generator causes the externally toothed gear to flex into an
ellipsoidal shape so that the first external teeth are caused to
mesh with the first internal teeth at two positions along a
circumferential direction and the second external teeth are caused
to mesh with the second internal teeth at two positions along the
circumferential direction; and a difference between a number of the
first external teeth and a number of the second external teeth is
2n, where n is a positive integer.
Description
TECHNICAL FIELD
The present invention relates to a strain wave gearing which has a
pair of internally toothed gears, a cylindrical externally toothed
gear capable of flexing in a radial direction, and a wave
generator.
BACKGROUND ART
Strain wave gearings having cylindrical externally toothed gears
are typically provided with a stationary-side internally toothed
gear secured so as not to rotate, a wave generator that is a
rotation-inputting element, a drive-side internally toothed gear
that is a reduced-rotation-outputting element, and a cylindrical
externally toothed gear capable of flexing in the radial direction
and meshing with the stationary-side internally toothed gear and
drive-side internally toothed gear. In typical strain wave
gearings, the externally toothed gear is caused to flex into an
ellipsoidal shape, the ellipsoidally flexed externally toothed gear
meshing with the stationary-side and drive-side internally toothed
gears at both end positions along the major axis of the ellipsoidal
shape.
Patent Documents 1 discloses typical strain wave gearings in which
the number of teeth of the stationary-side internally toothed gear
is two greater than that of the externally toothed gear, and the
number of teeth of the drive-side internally toothed gear is equal
to that of the externally toothed gear. The external teeth of the
externally toothed gear are bisected at the tooth-trace-direction
central portion thereof, one of the external-tooth portions being
capable of meshing with the stationary-side internally toothed
gear, and the other of the external-tooth portions being capable of
meshing with the drive-side internally toothed gear. When the wave
generator rotates, the externally toothed gear rotates more slowly
at a speed ratio corresponding to the difference in the number of
teeth with respect to the stationary-side internally toothed gear.
The reduced rotation of the externally toothed gear is outputted
from the drive-side internally toothed gear, which rotates
integrally with the externally toothed gear.
Strain wave gearing are characterized in which it is possible to
have high reduction ratios and a high response without backlash.
However, in some cases in which strain wave gearings having a low
reduction ratio are desirable. In the strain wave gearings, when
the speed ration thereof is made small, the radial flexing amount
of the externally toothed gear thereof becomes large. In the
consideration of mechanical characteristics, mechanical
performances and other factors such as the flexible externally
toothed gear meshes with the internally toothed gear while it is
being flexed, typical strain wave gearings have a speed ratio of 50
or higher, and it is difficult for the strain wave gearings to have
a speed ratio as low as 20 to 50.
Patent Document 2 discloses a strain wave gearing in which the
number of teeth of the stationary-side internally toothed gear is
two greater than that of the externally toothed gear, and the
number of teeth of the drive-side internally toothed gear is two
less than that of the externally toothed gear. In this strain wave
gearing, when the wave generator rotates, the externally toothed
gear rotates more slowly at a speed ratio corresponding to the
difference in the number of teeth with respect to the
stationary-side internally toothed gear. The rotation of the
externally toothed gear is increased at a speed ratio corresponding
to the difference in number of teeth between the externally toothed
gear and the drive-side internally toothed gear, and is outputted
from the drive-side internally toothed gear. The rotation outputted
from the drive-side internally toothed gear is reduced at a speed
ratio of less than 50 in relation to the rotation inputted to the
wave generator.
Patent Documents 2 and 3 disclose strain wave gearings having wave
generators that have two rows of ball bearings. This type of wave
generator is configured from a rigid plug having an ellipsoidally
contoured outer-peripheral surface, and two rows of ball bearings
fitted to the outer-peripheral surface. The flexible externally
toothed gear is pressed radially outward by the two major-axis end
portions of the outer-peripheral surfaces of the ellipsoidally
flexed outer races of the ball bearings, and the meshing of the
flexible externally toothed gear with respect to the first and
second rigid internally toothed gears is sustained.
PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP-A 2011-112214
Patent Document 2: JP-A 02-275147
Patent Document 3: JP-U 01-91151
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
It is considered that in the externally toothed gear used herein,
first teeth capable of meshing with one first internally toothed
gear and second teeth capable of meshing with another second
internally toothed gear are formed in the outer-peripheral surface
of a radially flexible cylindrical body, the second teeth differing
in number from the first teeth. Adopting such a configuration makes
it possible to realize a strain wave gearing having a speed ratio
of less than 50 in a similar manner as in the strain wave gearing
disclosed in Patent document 2. Additionally, this configuration
enables a strain wave gearing having a speed ratio of less than 50
to be designed with a greater degree of freedom than in the strain
wave gearing disclosed in Patent Document 2.
In the present specification, a strain wave gearing that has an
externally toothed gear in which first and second external teeth
differing in number are formed in the outer-peripheral surface of a
flexible cylindrical body is referred to as a "dual-type strain
wave gearing." The dual-type strain wave gearing has problems
described hereinafter.
First, in a dual-type strain wave gearing, first external teeth and
second external teeth of an externally toothed gear are formed in
the outer-peripheral surface of a shared cylindrical body, and the
tooth bottom rim parts of the first and second external teeth are
connected to each other. The first and second external teeth
differing in number are made to mesh with different internally
toothed gears, respectively, so that the force applied on the first
external teeth caused by meshing with the internal teeth of one
internally toothed gear is largely different from the force applied
on the second external teeth caused by meshing with the internal
teeth of the other internally toothed gear. Specifically, different
from the case in which the external teeth are separated into two
parts along the tooth trace direction, since the first external
teeth and the second external teeth differ in number, the tooth
profiles of these two external teeth differ as well.
Accordingly, high stress concentration and large amount of torsion
occur in portions between the first and second external teeth that
are formed in the outer peripheral surface of the flexible
cylindrical body formed from a thin-walled elastic body. As a
result, in the first and second external teeth, their tooth-contact
states with respect to the internal teeth at each point along the
tooth trace direction are changed, and their tooth-land load
distributions along the tooth trace direction are greatly
fluctuated.
When the tooth-contact states are changed and the tooth-land load
distributions are greatly fluctuated, the tooth bottom fatigue
strength and load transfer torque of the externally toothed gear
cannot be increased. In order to increase the tooth bottom fatigue
strength and load transfer torque of the externally toothed gear,
it is necessary that the tooth-contact distribution be made uniform
so as to decrease the maximum tooth-contact load, and that tooth
contact on each point along the tooth trace direction be kept in a
suitable manner.
Further, the meshing states of the first and second external teeth
with the respective internal teeth, especially their
tooth-trace-direction meshing states are affected by the support
rigidity of the wave generator. When the meshing states along the
tooth trace direction are not appropriate, the transfer load torque
is decreased.
Therefore, in order to increase the tooth bottom fatigue strength
and transfer load torque of the externally toothed gear, it is
necessary that the tooth-land load distribution be made uniform so
as to reduce the maximum tooth-land load, and that tooth contact at
each point along the tooth trace direction be kept in a suitable
state. Further, in order to maintain an appropriate meshing state
at each point along the tooth trace direction, the support rigidity
of the wave generator must be increased.
Furthermore, if the externally toothed gear is not appropriately
supported by the wave generator, the bearing-ball load
distributions appeared in the two row of the ball bearings of the
wave generator become uneven, whereby the lifetime of the bearings
is shortened. Therefore, for the purpose that the bearing-ball load
distribution is made even and durability of the bearings is
enhanced, it is necessary to support the meshing portions between
the first external teeth and the internal teeth of one internally
toothed gear, and the meshing portions between the second external
teeth and the internal teeth of the other internally toothed gear
in an appropriate matter.
In view of the above point, an object of the present invention is
to provide a dual-type strain wave gearing which can easily realize
a low speed ratio, has an increased tooth bottom fatigue strength
of the flexible externally toothed gear, and has a large load
capacity.
In addition to the above object, another object of the present
invention is to provide a large-load-capacity dual-type strain wave
gearing which is provided with a wave generator having a high
durability and supporting the externally toothed gear with high
rigidity.
Means of Solving the Problems
In order to solve the problems described above, a dual-type strain
wave gearing of the present invention is characterized by
including:
a rigid first internally toothed gear in which first internal teeth
are formed;
a rigid second internally toothed gear in which second internal
teeth are formed, the second internally toothed gear being disposed
so as to be coaxially aligned in parallel with the first internally
toothed gear;
a flexible externally toothed gear in which first external teeth
capable of meshing with the first internal teeth and second
external teeth capable of meshing with the second internal teeth
are formed in an outer-peripheral surface of a radially flexible
cylindrical body, the second teeth differing in number from the
first teeth, and the externally toothed gear being disposed
coaxially inside the first and second internally toothed gears;
and
a wave generator which causes the externally toothed gear to flex
into an ellipsoidal shape, causing the first external teeth to
partially mesh with the first internal teeth and causing the second
external teeth to partially mesh with the second internal
teeth;
wherein a gap is formed between a tooth-trace-direction inner-end
surface of the first external teeth and a tooth-trace-direction
inner-end surface of the second external teeth, the gap having a
prescribed width along a tooth trace direction, and the gap having
a tooth-depth-direction deepest part at a tooth-trace-direction
central portion; the gap functions as a cutter clearance area for
tooth-cutting cutters used for cutting the first and second
external teeth;
a relationship 0.1L<L1<0.3L
is satisfied, where L is a width from a tooth-trace-direction outer
end of the first external teeth to a tooth-trace-direction outer
end of the second external teeth, and L1 is a tooth-trace-direction
maximum width of the gap; and
relationships 0.9h1<t1<1.3h1 and 0.9h2<t2<1.3h2
are satisfied, where h1 is a tooth depth of the first external
teeth, h2 is a tooth depth of the second external teeth, t1 is a
tooth-depth-direction depth from a tooth top land of the first
external teeth to the deepest part, and t2 is a
tooth-depth-direction depth from a tooth top land of the second
external teeth to the deepest part.
In the dual-type strain wave gearing, although the first external
teeth meshing with the first internal teeth and the second external
teeth meshing with the second internal teeth are connected at their
tooth bottom rim parts with each other, their tooth numbers and
modules differ to each other, and therefore their tooth profiles
differ to each other.
A speed ratio R1 between the first internally toothed gear and the
externally toothed gear having first external teeth, a speed ratio
R2 between the second internally toothed gear and the externally
toothed gear having second external teeth, and a speed ratio R of
the strain wave gearing are respectively defined as follows:
R1=1/{(Zf1-Zc1)/Zf1}, R2=1/{(Zf2-Zc2)/Zf2}, and
R=(R1.times.R2-R1)/(-R1+R2), where Zc1 is the tooth number of the
first internal teeth, Zc2 is the tooth number of the second
internal teeth, Zf1 is the tooth number of the first external
teeth, and Zf2 is the tooth number of the second external
teeth.
According to the strain wave gearing of the present invention, it
is possible to obtain a speed ratio of less than 50, e.g., a speed
ratio appreciably lower than 30. Additionally, unlike in the prior
art, first external teeth and second external teeth that differ in
number and module are formed as the external teeth of the
externally toothed gear. Accordingly, there is a greater degree of
freedom in the design for setting the speed ratio, and a strain
wave gearing having a low speed ratio can be realized more easily
than in the prior art.
Further, in the externally toothed gear of the dual-type strain
wave gearing of the present invention, different tooth cutters are
used to cut the first and second external teeth. For this reason,
the gap functioning as a cutter clearance area is formed in the
tooth-trace-direction central portion of the externally toothed
gear, namely, between the first and second external teeth.
The manner in which the gap is formed has a prominent effect on the
tooth contact of the first external teeth with respect to the first
internal teeth along the tooth trace direction, as well as the
tooth land load distribution. The manner in which the gap is formed
similarly has a prominent effect on the tooth contact of the second
external teeth with respect to the second internal teeth along the
tooth trace direction, as well as the tooth land load
distribution.
According to the present invention, in view of these points, the
maximum width L1 of the gap is set within a range of 0.1 to 0.3
times the width L of the externally toothed gear, and the maximum
depths t1 and t2 are set within a range of 0.9 to 1.3 times the
tooth depths h1, h2 of the first and second external teeth. It was
confirmed that forming the gap in this manner makes it possible to
maintain uniformity in the tooth-trace-direction tooth land load
distributions of the first and second external teeth and to
maintain a satisfactory state for the tooth contact of the first
and second external teeth with respect to the first and second
internal teeth at each tooth-trace-direction position.
According to the present invention, it is possible to realize a
strain wave gearing having a speed ratio less than 30, and to
realize a strain wave gearing having a high tooth bottom fatigue
strength and a high load capacity.
Next, the wave generator of the dual-type strain wave gearing of
the present invention has:
a first wave bearing comprising a ball bearing for supporting the
first external teeth, and a second wave bearing comprising a ball
bearing for supporting the second external teeth; and
bearing-ball centers of the first wave bearing and the second wave
bearing are located at positions that are equidistant, along the
tooth trace direction, from a tooth-trace-direction center of the
gap; and
wherein, where an inter-ball-center distance Lo is a distance
between the bearing-ball centers of the first and second wave
bearings,
the inter-ball-distance is set so as to increase correspondingly
with an increase in a maximum width L1 of the gap, and satisfies a
relationship 0.35L<Lo<0.7L.
In the prior art, a wave generator having two rows of ball bearings
is used in order to increase the area in which the externally
toothed gear is supported. The two rows of ball bearings were
arranged offset toward the tooth-width-direction central portion of
the externally toothed gear, without any consideration to the
inter-ball-center distance.
In the present invention, the inter-ball-center distance Lo between
two rows of wave bearings is increased such that it is possible to
increase rigidity for supporting first and second external teeth
differing in number, and to improve the tooth contact of each of
the external teeth with respect to internal teeth at each
tooth-trace-direction position. Specifically, a configuration is
adopted in which the inter-ball-center distance Lo lengthens
(increases) correspondingly with an increase in the
tooth-trace-direction maximum length L1 of the gap, which is formed
between the first and second external teeth and functions as a
cutter clearance area. The range of increase of the
inter-ball-center distance Lo is set to 0.35 to 0.7 times the width
L of the externally toothed gear.
According to the present invention, it is possible to arrange the
first and second wave bearings such that the ball centers are
positioned at suitable tooth-trace-direction positions with respect
to each of the first and second external teeth in accordance with
the width of the gap that is formed. This makes it possible to
reliably support the first and second external teeth, using the
first and second wave bearings, at each tooth-trace-direction
position of each of the first and second external teeth (i.e., to
increase the supporting rigidity of the wave generator).
As a result, it is possible to improve the tooth contact of the
first and second external teeth at each tooth-trace-direction
position, and to increase the tooth bottom fatigue strength
thereof. It is also possible to average the bearing-ball load
distribution of each of the wave bearings of the wave generator,
and to reduce the maximum load; therefore, the service life of the
wave generator can be improved.
In the dual-type strain wave gearing of the present invention,
generally, the number Zf1 of the first external teeth differs from
a number Zc1 of the first internal teeth, and the number Zf2 of
second external teeth differs from a number Zc2 of second internal
teeth. For example, the number Zf1 of first external teeth is less
than the number Zc1 of first internal teeth, and the number Zc1 of
first internal teeth and the number Zc2 of second internal teeth
are equal to each other.
In addition, the wave generator is set to be a rotation-inputting
element; and either one of the first internally toothed gear and
second internally toothed gear is set to be a stationary-side
internally toothed gear secured so as not to rotate, and the other
of the first internally toothed gear and second internally toothed
gear is a drive-side internally toothed gear that is a
reduced-rotation-outputting element.
Furthermore, the first and second external teeth of the externally
toothed gear are flexed into an ellipsoidal shape, a three-lobe
shape or other non-circular shape by the wave generator. This makes
the externally toothed gear to mesh with the rigid internally
toothed gear on plural positions apart from one another along the
circumferential direction. Typically, the externally toothed gear
is flexed into an ellipsoidal shape and is meshed with the
internally toothed gear on two positions apart from 180 degrees
along the circumferential direction (i.e. on both end positions of
the major axis of the ellipsoidal shape). In this case, the
differences between the tooth number Zf1 of the first external
teeth and the tooth number Zf2 of the second external teeth are set
to be 2n, where n is a positive integer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) and 1(b) are an end-surface views and a longitudinal
cross-sectional view of a dual-type strain wave gearing to which
the present invention is applied;
FIG. 2 is a schematic diagram of the dual-type strain wave gearing
shown in FIG. 1;
FIG. 3 is a partial enlarged cross-sectional view of the strain
wave gearing shown in FIG. 1;
MODE FOR CARRYING OUT THE INVENTION
An embodiment of a dual-type strain wave gearing to which the
present invention is applied is described below with reference to
the attached drawings.
FIG. 1 is an end-surface view and a longitudinal cross-sectional
view showing a dual-type strain wave gearing (referred to below
simply as "strain wave gearing") according to an embodiment of the
present invention, and FIG. 2 is a schematic diagram of the same.
The strain wave gearing 1, which is used as, e.g., a gear reducer,
has an annular rigid first internally toothed gear 2, an annular
rigid second internally toothed gear 3, a cylindrical flexible
externally toothed gear 4 comprising a radially flexible
thin-walled elastic body, and an ellipsoidally contoured wave
generator 5.
The first and second internally toothed gears 2, 3 are disposed so
as to be coaxially aligned in parallel with each other, with a
prescribed gap therebetween, along the direction of a central axis
1a. In the present example, the first internally toothed gear 2 is
a stationary-side internally toothed gear secured so as not to
rotate, the number of first internal teeth 2a thereof being
indicated by Zc1. The second internally toothed gear 3 is a
rotatably supported drive-side internally toothed gear, the number
of second internal teeth 3a thereof being indicated by Zc2. The
second internally toothed gear 3 is the reduced-rotation-outputting
element of the strain wave gearing 1.
The cylindrical externally toothed gear 4 is disposed coaxially
inside the first and second internally toothed gears 2, 3. The
externally toothed gear 4 has a cylindrical body 6 that is a
radially flexible thin-walled elastic body, first external teeth 7
and second external teeth 8 formed in the circular outer-peripheral
surface of the cylindrical body 6, and a gap 9 (refer to FIG. 3)
formed between the external teeth 7, 8 on either side, the gap 9
functioning as a cutter clearance area. The first external teeth 7
are formed on one side along the central axis 1a direction of the
circular outer-peripheral surface of the cylindrical body 6, and
the second external teeth 8 are formed on the other
second-internal-teeth 3a side of the circular outer-peripheral
surface. The first and second external teeth 7, 8 are formed such
that the central-axis 1a direction is the tooth trace
direction.
Specifically, the first external teeth 7 are formed on the side
opposing the first internal teeth 2a, and are capable of meshing
with the first internal teeth 2a, the number of first external
teeth 7 being indicated by Zf1. The second external teeth 8 are
formed on the side opposing the second internal teeth 3a, and are
capable of meshing with the second internal teeth 3a, the number of
second external teeth 8 being indicated by Zf2. The numbers Zf1,
Zf2 of teeth are different from each other. Further, the first
external teeth 7 and the second external teeth 8 are apart from
each other in the tooth-trace direction.
The wave generator 5 has an ellipsoidally contoured rigid plug 11,
and a first wave bearing 12 and second wave bearing 13, the first
and second wave bearings being fitted to the ellipsoidal
outer-peripheral surface of the rigid plug 11. The first and second
wave bearings 12, 13 are formed from ball bearings.
The wave generator 5 is inserted into the inner-peripheral surface
of the cylindrical body 6 of the externally toothed gear 4, and
causes the cylindrical body 6 to flex in an ellipsoidal shape.
Therefore, the first and second external teeth 7, 8 are also flexed
in an ellipsoidal shape. The ellipsoidally flexed externally
toothed gear 4 meshes with the first and second internally toothed
gears 2, 3 at both end positions along the major axis Lmax of the
ellipsoidal shape. Specifically, the first external teeth 7 mesh
with the first internal teeth 2a at both end positions along the
major axis of the ellipsoidal shape, and the second external teeth
8 mesh with the second internal teeth 3a at both end positions
along the major axis.
The wave generator 5 is the rotation-input element of the strain
wave gearing 1. The rigid plug 11 of the wave generator 5 has a
shaft hole 11c, in which an input rotation shaft 10 (refer to FIG.
2) is securely connected in a coaxial arrangement. For example, a
motor output shaft may be securely connected in a coaxial
arrangement in the shaft hole 11c. When the wave generator 5
rotates, the positions at which the first external teeth 7 of the
externally toothed gear 4 and the stationary-side first internal
teeth 2a mesh, and the positions at which the second external teeth
8 of the externally toothed gear 4 and the drive-side second
internal teeth 3a mesh, move along the circumferential
direction.
The number Zf1 of first external teeth 7 and the number Zf2 of
second external teeth 8 differ from each other; in the present
example, the number Zf2 of second external teeth is greater. The
number Zc1 of first internal teeth 2a and the number Zf1 of first
external teeth 7 also differ from each other; in the present
example, the number Zc1 of first internal teeth 2a is greater. The
number Zc2 of second internal teeth 3a and the number Zf2 of second
external teeth 8 differ from each other; in the present example,
the number Zc2 of second internal teeth 3a is less.
In the present example, the externally toothed gear 4 is caused to
flex in an ellipsoidal shape, and meshes with the internally
toothed gears 2 and 3 at two locations along the circumferential
direction. Therefore, the difference between the number Zc1 of
first internal teeth 2a and the number Zf1 of first external teeth
7 is 2j, where j is a positive integer. The difference between the
number Zc2 of second internal teeth 3a and the number Zf2 of second
external teeth 8 is 2k, where k is a positive integer. Zc1=Zf1+2j
Zc2=Zf2-2k
In a specific example, the numbers of teeth are set as follows
j=k=1: Zc1=62 Zf1=60 Zc2=62 Zf2=64
The speed ratio R1 between the first internally toothed gear 2 and
the first external teeth 7, and the speed ratio R2 between the
second internally toothed gear 3 and the second external teeth 8,
are respectively defined as follows:
i1=1/R1=(Zf1-Zc1)/Zf1=(60-62)/60=-1/30
i2=1/R2=(Zf2-Zc2)/Zf2=(64-62)/64=1/32
Therefore, R1=-30, and R2=32.
The speed ratio R of the strain wave gearing 1 is represented by
the following formula using the speed ratios R1, and R2. Thus,
according to the present invention, a strain wave gearing having a
dramatically low speed ratio (low reduction ratio) can be realized
(a negative speed ratio indicates that output rotation progresses
in the direction opposite that of input rotation).
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times. ##EQU00001##
(Gap: Cutter Clearance Area)
FIG. 3 is a partial enlarged cross sectional view of the strain
wave gearing, which shows the externally toothed gear 4 as well as
the first and second wave bearings 12 and 13 of the wave generator
5. The gap 9 formed between the first and second external teeth 7
and 8 functions as a cutter clearance area for tooth-cutting
cutters used for cutting the first and second external teeth 7 and
8.
The first and second external teeth 7 and 8 will be explained at
first. Since the first and second internal teeth 2a and 3a has
substantially the same tooth width, the first external teeth 7 and
the second external teeth 8 having the same tooth width are formed
in a symmetrical state with respect to the tooth-trace-direction
central position 6a of the cylindrical body 6. When the first and
second internal teeth differ in tooth width with each other, the
first and second external teeth 7 and 8 will correspondingly differ
in tooth width.
The gap 9 has a prescribed width along the tooth trace direction;
the deepest part, which is the part of the gap 9 that is formed
deepest along the tooth depth direction, is formed in the
tooth-trace-direction central portion. In the present example, the
deepest part 9a is a portion at which the tooth-trace-direction
central portion is defined by a straight line extending parallel to
the tooth trace direction, as viewed from the tooth-thickness
direction. At the two tooth-trace-direction ends of the deepest
part 9a, a concave arcuate curve that defines the
tooth-trace-direction inner-end surface 7a of the first external
teeth 7 and a concave arcuate curve that defines the
tooth-trace-direction inner-end surface 8a of the second external
teeth 8 are smoothly connected. It is also possible to adopt a
configuration in which the deepest part 9a is defined by a concave
curved surface and the two inner-end surfaces 7a, 8a are defined by
inclined straight lines. It is furthermore possible to adopt a
configuration in which the deepest part 9a is defined by a straight
line and the two inner-end surfaces 7a, 8a are defined by inclined
straight lines.
The tooth-trace-direction width of the gap 9 in the present example
gradually increases from the deepest part 9a along the tooth depth
direction. The maximum width L1 in the tooth trace direction is the
distance, along the tooth trace direction, from the
tooth-trace-direction inner end 7b of the addendum circle of the
first external teeth 7 to the tooth-trace-direction inner end 8b of
the addendum circle of the second external teeth 8.
The relationship 0.1L<L1<0.3L
is established, where L is the width from the tooth-trace-direction
outer end 7c of the first external teeth 7 to the
tooth-trace-direction outer end 8c of the second external teeth 8,
and L1 is the tooth-trace-direction maximum width of the gap 9.
The depth of the deepest part 9a of the gap 9 is set as follows.
The relationships 0.9h1<t1<1.3h1 and 0.9h2<t2<1.3h2
are established, where h1 is the tooth depth of the first external
teeth 7, h2 is the tooth depth of the second external teeth 8, t1
is the tooth-depth-direction depth from the top land 7d of the
first external teeth 7 to the deepest part 9a, and t2 is the
tooth-depth-direction depth from the top land 8d of the second
external teeth 8 to the deepest part 9a.
[Distance Between Bearing-Ball Centers]
The distance between the bearing-ball centers of the first and
second wave bearings 12, 13 are described next with reference to
FIG. 3.
In the rigid plug 11 of the wave generator 5, an ellipsoidally
contoured first outer-peripheral surface 11a of fixed width is
formed on one central-axis-direction side, and an ellipsoidally
contoured second outer-peripheral surface 11b of fixed width is
formed on the other central-axis-direction side. The first
outer-peripheral surface 11a and the second outer-peripheral
surface 11b are ellipsoidal outer-peripheral surfaces having the
same shape and the same phase. The first and second
outer-peripheral surfaces 11a and 11b may be different ellipsoidal
shapes in accordance with the difference in the amount of
deflection between the first and second external teeth 7 and 8.
The first wave bearing 12 is fitted to the first outer-peripheral
surface 11a in a state of being flexed in an ellipsoidal shape, and
the second wave bearing 13 is fitted to the second outer-peripheral
surface 11b in a state of being flexed in an ellipsoidal shape. The
first and second wave bearings 12, 13 are of the same size.
The bearing centers 12a, 13a of the first wave bearing 12 and
second wave bearing 13 are located at positions that are
equidistant, along the tooth width direction, from the
tooth-trace-direction central position 6a on the externally toothed
gear 4. The distance between bearing-ball centers is set so as to
increase correspondingly with an increase in the maximum width L1
of the gap 9. Furthermore, the inter-ball-center distance Lo is set
so as to reach a value within the range indicated by the following
formula, Lo being the distance between bearing-ball centers.
0.35L<Lo<0.7L
Other Embodiments
In the example described above, the first internally toothed gear 2
is configured as a stationary-side internally toothed gear, and the
second internally toothed gear 3 is configured as a drive-side
internally toothed gear. It is possible to instead configure the
first internally toothed gear 2 as a drive-side internally toothed
gear, and configure the second internally toothed gear 3 as a
stationary-side internally toothed gear.
It is also possible to flex the externally toothed gear 4 into a
non-circular shape other than an ellipsoidal shape, for example,
into a non-circular shape such as a three-lobe shape. When h
represents the number of meshing positions between the externally
toothed gear flexed into a non-circular shape and the internally
toothed gear, the difference in the number of teeth between the two
gears may be set hp, where h is a positive integer equal to or more
than 2, and p is a positive integer.
* * * * *